Method for producing isocynates

ABSTRACT

The invention relates to a process for the production of isocyanates by reaction of primary amines with phosgene, in which isocyanate is used as solvent, wherein some or all of isocyanate used as solvent is added to the reaction mixture only after the amine and phosgene have been physically combined.

DESCRIPTION

[0001] The present invention relates to a process for the production ofisocyanates by reaction of amines with phosgene using isocyanate assolvent, the isocyanate used as solvent being added to the reactionmixture only after the amine and phosgene have been physically combined.

[0002] Various processes for the production of isocyanates by reactionof amines with phosgene in the presence of isocyanate are known.

[0003] DE-A 1,192,641 describes a process for the production ofisocyanates by reaction of amines with phosgene, the isocyanate producedduring the reaction being used as solvent for phosgene.

[0004] DE-A 2,252,068 describes a process for the production of organicisocyanates, wherein amine is caused to react with preheated phosgene inthe presence of an excess of isocyanate at temperatures and pressuresselected so as to give a homogeneous liquid phase.

[0005] U.S. Pat. No. 2,822,373 describes a continuous process for theproduction of isocyanates, in which a phosgene solution is mixed with asolution of an organic amine in a turbulent reactor circulation system.In this case, the fresh phosgene solution is combined with thecirculated reaction solution prior to blending with the amine solution.

[0006] WO 96/16028 describes a continuous process for the production ofisocyanates by reaction of corresponding primary amines with phosgene inthe presence of an isocyanate acting as solvent, wherein the amine iscaused to react with phosgene, which is dissolved in the isocyanate in aconcentration of from 10 to 60 wt %.

[0007] It is also known that the use of a high phosgene excess over theamino groups used leads to high selectivities toward the isocyanateproduced and can thus have a decisive influence on the economical valueof the process. However, with an increasing ratio of phosgene to aminogroups there is a rise in the phosgene hold-up of the plant, whichshould, however, be minimized on account of the toxicity of phosgene.

[0008] It is thus an object of the present invention to provide aprocess for the production of isocyanates which makes it possible tocarry out the reaction with high selectivity without raising thephosgene hold-up or lowering the space-time yield.

[0009] The object of the invention is achieved, unexpectedly, in theproduction of isocyanates by the reaction of amine with phosgene, byadding an isocyanate as solvent, which solvent is added not, asdescribed in the prior art, to the phosgene prior to the reaction, butto the reaction mixture formed by physically combining the phosgene andamine.

[0010] Thus the present invention relates to a process for theproduction of isocyanates by reaction of primary amines with phosgene,in which isocyanate is used as solvent, wherein some or all of theisocyanate used as solvent is added to the reactants only when the amineand phosgene have been physically combined.

[0011] The process of the invention includes continuous, semicontinuous,and batch processes. Preference is given to continuous processes. Duringthe production of isocyanate by reaction of a primary amine withphosgene there is formed, in an initial fast step, according tofollowing reaction scheme, the intermediate carbamoyl chloride, whichdecomposes in the rate-determining, slow step in a balanced reaction toform isocyanate and HCl.

R—NH₂+COCl₂→R—NH—COCl+HCl

R—NCO+2 HCl,

[0012] in which R is an organic radical.

[0013] Furthermore the resulting hydrogen chloride can react with aminesto form amine hydrochlorides.

[0014] The essential feature of the process of the invention is that theaddition of the isocyanate acting as solvent does not occur until theamine and phosgene have been physically combined, preferably by mixing.In one embodiment of the invention, the solvent is added at a time orpoint at which at least 50%, preferably at least 80%, more preferably atleast 90%, and most preferably at least 95% of the amino groups usedhave already undergone reaction.

[0015] The rate of the reaction between phosgene and amine or betweenhydrogen chloride and amine is primarily governed by the type ofisocyanate to be synthesized and the reaction temperature used.Correspondingly, the addition of isocyanate acting as solvent can takeplace some time between 0.1 milliseconds and 10 minutes followingphysical combination of phosgene and amine.

[0016] In the process of the invention, some or all of the isocyanateused as solvent is added to the reaction mixture, ie the reactants amineand phosgene, only after the amine and phosgene have been physicallycombined. In a preferred embodiment, the portion added to the reactionmixture after the amine and phosgene have been physically combined is atleast 25 wt %, preferably at least 50 wt %, more preferably at least 75wt %, and most preferably at least 90 wt %, based on the total amount ofisocyanate to be used as solvent. The portion thereof which is not to beadded after the amine and phosgene have been physically combined, can beadded prior to the reaction either to the amine or, preferably, to thephosgene.

[0017] In the process of the invention, use can be made of any oneprimary amine or a mixture of two or more such amines. Preference isgiven to aromatic amines, particularly those in thediaminodiphenylmethane series or their higher homologs. Examples thereofare methylenediphenylamine (MDA; individual isomers, mixture of isomersand/or oligomers thereof), toluylenediamine (TDA), n-pentylamine,6-methyl-2-aminoheptane, cyclopentylamine, R,S 1-phenylethylamine,1-methyl-3-phenylpropylamine, 2,6-xylidine,2-(N,N-dimethylamino)ethylamine, 2-(N,N-diisopropylamino)ethylamine,C11-neodiamine, isophoronediamine, 3,3′-diaminodiphenylsulfone, and4-aminomethyl-1,8-octanediamine. MDA and TDA are preferably used.

[0018] The process of the invention is thus suitable for use in thesynthesis of any desired isocyanates. The process can be used withparticular advantage for the production of methylene(diphenyl-diisocyanate) (MDI) and toluylene-diisocyanate (TDI).

[0019] The isocyanate used as solvent is preferably the isocyanate to besynthesized. It may come from an external source or, preferably, betaken from the process of the invention and recycled. Alternativelyhowever, other suitable isocyanates, or mixtures thereof, may be used assolvent.

[0020] An additional inert solvent can be co-used in the process of theinvention. This additional inert solvent is usually an organic solventor a mixture thereof. Chlorobenzene, dichlorobenzene, trichlorobenzene,toluene, hexane, diethyl isophthalate (DEIP), tetrahydrofuran (THF),dimethylformamide (DMF), benzene, and mixtures thereof are preferred.Particular preference is given to chlorobenzene. The additional inertsolvent can be added to the amine preferably at the commencement of thereaction. The inert solvent is usually employed in a concentration offrom 5 to 1000 wt %, and preferably from 50 to 500 wt %, based on theamount of amine used.

[0021] The process of the invention will now be described in detail withreference to a general flow sheet illustrating a continuous process, asshown in FIG. 1. The elements depicted in FIG. 1 are as follows:

[0022] I phosgene receiver

[0023] II amine receiver

[0024] III first mixing device

[0025] IV second mixing device

[0026] V reaction device

[0027] VI first separating device

[0028] VII second separating device

[0029] VIII isocyanate feedstock

[0030] IX phosgene purifier

[0031] X solvent purifier

[0032]1 phosgene feedline

[0033]2 amine feedline

[0034]3 inert solvent feedline

[0035]4 separated hydrogen chloride and inert solvent

[0036]5 recycled isocyanate stream

[0037]6 discharged hydrogen chloride

[0038]7 separated isocyanate

[0039]8, 11 separated inert solvent

[0040]9 purified inert solvent

[0041]10 purified phosgene

[0042] The amine from amine receiver II and phosgene from phosgenereceiver I are mixed in a suitable first mixing device III. The mixtureof amine and phosgene is mixed, in a second mixing device IV, withisocyanate acting as solvent and passed to the reaction device V.Suitable mixing devices are, for example, nozzles or blender reactors.It is also possible to carry out the two mixing operations of mixingdevices III and IV in a common mixing device but in discrete regions, asexplained below with reference to the preferred embodiment illustratedin FIG. 2.

[0043] After mixing, the mixture is passed to a reaction device V.Suitable reaction devices are, for example, tubular reactors, towerreactors, reaction vessels or reaction columns. Tubular reactors arepreferred. Also useful are contrivances which are both mixer andreactor, for example, tubular reactors having flanged-on nozzles.

[0044] The two separating devices VI and VII are preferably distillationunits. In the first separating device VI, hydrogen chloride and,optionally, inert solvent and/or small portions of the isocyanate streamare usually separated from the isocyanate stream. In the secondseparating device VII, preferably inert solvent is separated and thenpurified (X) and recycled to amine receiver II.

[0045] A preferred embodiment of the process of the invention involves,as illustrated in FIG. 2, the use of a mixing device described below,which is directly followed by a reaction device.

[0046]FIG. 2 illustrates a preferred assembly for the process of theinvention. The elements depicted in FIG. 2 are as follows:

[0047]1 amine feed

[0048]2 phosgene feed

[0049]3 isocyanate feed

[0050]4 a injector tube

[0051]4 b injector tube

[0052]5 diffuser

[0053]6 tubular reactor

[0054]7 axis

[0055]8 annular gap in the phosgene feedline

[0056]9 annular gap in the isocyanate feedline

[0057] In continuous phosgenation, the amine or the amine solution 1 isinjected along the axis 7 into the mixing device, usually at rates offrom 5 to 60 m/s. The phosgene or phosgene solution passes into themixing device through an annular gap in the phosgene feedline 8 at ratesalso ranging from 5 to 60 m/s. The two streams of amine and phosgene arephysically combined (corresponding to the first mixing device in FIG. 1)and passed through an optional injector tube 4 a, after which isocyanateis fed in through annular gap 9 in the isocyanate feedline(corresponding to the second mixing device in FIG. 1). After passingthrough an optional injector tube 4 b, the reaction mixture passesthrough diffuser 5 into tubular reactor 6. Following a residence time offrom 10 s to 20 min, the resulting crude isocyanate solution is removedfrom the tubular reactor.

[0058] In a preferred embodiment, the injector mixing device used is anaxially symmetrical injector tube device having an axial amine feed andphosgene and isocyanate feeds effected via two off-axis annular gaps.

[0059] The optimal temperature range for the process of the invention isgoverned, inter alia, by the type and concentration of the solvent andby the isocyanate to be synthesized. Generally, the temperature in themixing unit is between −20° C. and 300° C., preferably between 10° C.and 200° C., and more preferably between 80° C. and 150° C. Thetemperature in the reactor is generally between 10° C. and 360° C.,preferably between 40° C. and 210° C., and more preferably between 100°C. and 180° C. Furthermore, the absolute pressure is generally between0.2 bar and 50 bar, preferably between 1 bar and 25 bar, and morepreferably between 3 and 17 bar.

[0060] The residence time of the fluid in the mixing device and thereactor is, in all, between 12 s and 20 min, preferably in the range offrom 36 s to 16 min, and more preferably between 60 s and 12 min.

[0061] The molar ratio of phosgene used to amino groups is from 1:1 to12:1, preferably from 1.1:1 to 4:1. Amine and phosgene can be used freefrom solvent or dissolved in one or more of the aforementioned inertsolvents. Alternatively, the phosgene can be injected as a gas into theamine solution. In addition, the phosgene may be premixed with a portionof the isocyanate used as solvent, as described above.

[0062] The amount of isocyanate used as solvent in the process of theinvention is generally from 10 to 1000 wt %, preferably from 50 to 500wt %, and more preferably from 100 to 400 wt %, based on the amount ofphosgene used.

[0063] Following the reaction, the mixture of substances is separatedinto isocyanate, solvent, phosgene, and hydrogen chloride, preferably bymeans of rectification. Small amounts of by-products remaining in theisocyanate can be separated from the desired isocyanate by means ofadditional rectification or, alternatively, by crystallization.

[0064] Depending on the reaction conditions chosen, the crude endproduct may contain inert solvent, carbamoyl chloride, and/or phosgeneand can be further processed by known methods (cf, eg, WO 99/40059).Furthermore, it may be advantageous to pass the product over a heatexchanger after discharge.

[0065] The invention is illustrated below by the following examples.

EXAMPLE 1

[0066] 90 g of a toluylenediamine mixture (TDA mixture), comprising 80wt % 2,4-TDA and 20 wt % 2,6-TDA was dissolved in 360 g ofmonochlorobenzene (MCB). The resulting TDA solution was injected intothe mixing device illustrated in FIG. 2 along the axis at a rate of 30m/s and a throughput of 1.8 L/h, at a temperature of 50° C.Simultaneously, 1.6 kg of a 25% strength phosgene solution, which hadbeen produced from 400 g of phosgene and 1.2 kg of MCB, were injected ata temperature of 30° C. through the first annular gap of the nozzlemixer at a rate of 30 m/s and at an angle of 45° to the axis. Then 1.2kg of toluylene-diisocyanate, comprising 80 wt % 2,4-TDI and 20 wt %2,6-TDI, were injected at a temperature of 30° C. into the axialinjector tube through the second annular gap at an angle of 900 to theaxis of the mixing device and at a rate of 25 m/s in the plane of entry.The mixture passed directly from the nozzle mixer into a tubularreactor, which had a capacity of ca 300 mL and a length of 24 m andprovided a residence time for the total mixture of ca 1.4 min and waskept at a temperature of from ca 130° to 140° C. Before and aftersynthesis, the apparatus was continuously rinsed with monochlorobenzeneas inert solvent. Following removal, by distillation, of the phosgeneand chlorobenzene, TDI was isolated at a purity of ca 99.2% (GC) and ayield of 99.1%. The phosgene hold-up in the reactor was, based on theincoming phosgene mass flow, not more than ca 38 g. This value wascalculated on the assumption that phosgene was not consumed during thereaction. This gave a phosgene hold-up of ca 76 g for a productionoutput of 1 kg of TDI per hour.

COMPARATIVE EXAMPLE 2 Using an Isocyanate/phosgene Solution

[0067] In a manner similar to that described in Example 1, 90 g of atoluylenediamine mixture (TDA mixture) comprising 80 wt % 2,4-TDA and 20wt % 2,6-TDA were dissolved in 360 g of monochlorobenzene. The resultingTDA solution was injected into the mixing device illustrated in FIG. 2along the axis at a rate of 30 m/s and a throughput of 1.8 L/h, at atemperature of 50° C. Simultaneously, 1.6 kg of a 25% strength phosgenesolution, which had been produced from 400 g of phosgene and 1.2 kg oftoluylene diisocyanate, comprising 80 wt % 2,4-TDI and 20 wt % 2,6-TDI,were injected at a temperature of 30° C. through the first annular gapof the nozzle mixer at a rate of 30 m/s and at an angle of 45° to theaxis. The mixture passed directly from the nozzle mixer into a tubularreactor, which had a capacity of ca 180 mL and a length of 14.4 m andprovided a residence time for the total mixture of ca 1.4 min and waskept at a temperature of from ca 130° to 140° C. Before and aftersynthesis, the apparatus was continuously rinsed with monochlorobenzeneas inert solvent. Following removal, by distillation, of the phosgeneand chlorobenzene, TDI was isolated at a purity of ca 99.1% (GC) and ayield of 98.5%. The phosgene hold-up in the reactor was, based on theincoming phosgene mass flow, not more than ca 38 g. This value wascalculated on the assumption that phosgene was not consumed during thereaction. This gave a phosgene hold-up of ca 76 g for a productionoutput of 1 kg of TDI per hour.

COMPARATIVE EXAMPLE 3 Using a MCB-phosgene Solution

[0068] In a manner similar to that described in Example 1, 90 g of atoluylenediamine mixture (TDA mixture) comprising 80 wt % 2,4-TDA and 20wt % 2,6-TDA were dissolved in 360 g of monochlorobenzene. The resultingTDA solution was injected into the mixing device illustrated in FIG. 2along the axis at a rate of 30 m/s and a throughput of 1.8 L/h, at atemperature of 50° C. Simultaneously, 1.6 kg of a 25% strength phosgenesolution, which had been produced from 400 g of phosgene and 1.2 kg ofMCB, were injected at a temperature of 30° C. through the first annulargap of the nozzle mixer at a rate of 30 m/s and at an angle of 45° tothe axis. The mixture passed directly from the nozzle mixer into atubular reactor, which had a capacity of ca 375 mL and a length of 30 mand provided a residence time for the total mixture of ca 2.7 min andwas kept at a temperature of from ca 130° to 140° C. Before and aftersynthesis, the apparatus was continuously rinsed with monochlorobenzeneas inert solvent. Following removal, by distillation, of the phosgeneand chlorobenzene, TDI was isolated at a purity of ca 99.1% (GC) and ayield of 97.4%. The phosgene hold-up in the reactor was, based on theincoming phosgene mass flow, not more than ca 72 g. This value wascalculated on the assumption that phosgene was not consumed during thereaction. This gave a phosgene hold-up of ca 147 g for a productionoutput of 1 kg of TDI per hour.

SUMMARY OF THE EXAMPLES

[0069] Table 1 below compares the results of Example 1 and ComparativeExamples 2 and 3. Table 1 clearly shows that the process of theinvention leads to a rise in selectivity without raising the phosgenehold-up or lowering the space-time yield. Comparative ComparativeExample Example 1 Example 2 Example 3 Phosgene hold-up for a 76 g 76 g147 g production output of 1 kg/h Yield 99.2% 98.5% 97.4 Distillationpurity 99.1% 99.1% 99.1%

1. A process for the production of isocyanates by reaction of primaryamines with phosgene, in which isocyanate is used as solvent, whereinsome or all of the isocyanate used as solvent is added to the reactantsonly after the amine and phosgene have been physically combined.
 2. Aprocess as defined in claim 1, which is carried out continuously.
 3. Aprocess as defined in claim 1, wherein portions of the isocyanateproduced are recycled and used as solvent.
 4. A process as defined inany of claims 1 to 3, wherein the amount of isocyanate added to thereaction mixture only after the amine and phosgene have been physicallycombined is at least 25 wt %, based on the total amount of isocyanateused as solvent.
 5. A process as defined in any of claims 1 to 4,wherein the amine used is methylenedi(phenylamine) or toluylenediamine.6. A process as defined in any of claims 1 to 5, wherein the reaction ofamine with phosgene is carried out in a tubular reactor having anupstream injector mixing device.
 7. A process as defined in claim 6,wherein the injector mixing device used is an axially symmetricalinjector tube device having an axial amine feed and phosgene andisocyanate feeds effected through two off-axis annular gaps.